Extinct Hydrochoerinae traditionally included within “Cardiomyinae” (Cavioidea, Caviidae) are caviomorph rodents well-represented in the late Miocene-late Pliocene of Argentina, but their paleobiology has been investigated little. The postcranium of these rodents is poorly-known and has not been considered in morpho-functional or systematic studies. Here, we provide the first description of the postcranium of the basal hydrochoerine Cardiomys Ameghino, 1885, based on a well preserved specimen from the late Miocene of Central Argentina, and evaluate its paleobiological and systematic implications. A morpho-functional study and a character mapping analysis were performed. We concluded that most of its postcranial features are neither adaptations to a specialized cursoriality, as in some extant cavioids, nor major modifications associated with swimming, as in extant capybaras. Cardiomys exhibits several features (high humeral distal articular surface, perforated olecranon fossa, proximal radius cranially located with respect to the ulna, subrectangular-shaped radial head with flattened ulnar facet, calcaneocuboid joint distally located with respect to the astragalonavicular joint) that allow us to consider it as an ambulatory caviid. Among cavioids, some features of Cardiomys are more similar to those of Hydrochoerus Brisson, 1762 (lateral coronoid process reduced, humeral capitular tail well-differentiated, capitular tail facet of the radial head well-developed and relatively short craniodistally, plantar process of the navicular massive and short). Other postcranial features (relatively longer and more gracile Mt III and phalanges, straight caudal border of the ulna) suggest that Cardiomys would have been a generalized hydrochoerine, as was also indicated by its dental and cranial characters.